Electrical resistance unit



March 5,1929. l 15E-ALLEN l 1,704,159

ELECTRICAL RESISTANG UNIT Filed Feb. 4, 1925 34--1 y Fl'gl [3 Il" 17u. 30 7 a' .0 5' 5,

IN V EN TOR.

BWM

A TTORNEY Patented. Mar.v .5, 1929.

UNITED STATES STRATFORD B. lALLEN', OF CAMBRIDGE, MASSACHUSETTS.

ELECTRICAL RESISTANCE UNIT.

Application filed February 4, 1925. Serial No. 6,732.

The invention relates to improvements in electrical resistance units. More particularly it relates to rheostats of the compression type, which may be for example of the order of one or more megohms, or may be of very much less resistance even down to a few ohms only. The invention has been particularly devel- ,oped for the resistances of high order first mentioned. It may therefore conveniently be described and illustrated as it may be applied for use as a grid leak used in connection with a thermionic tube orthe like in radio receiving apparatus. It may, however,n be put to other uses; and when constructed in a dilferent order of values of resistance, suitable for the purpose, it may be applied as a means for controlling volume in audio frequency amplifiers, or in the primary or in the secondary of a transformer or in any of a wide variety of other situations.

It is a particular object of the invention to provide a grid leak which is adjust-able through a.wide range; and to provide a construction wherein the adjustment is easily made, the materials and their processes of manufacture simple and inexpensive, and the accuracy of the resulting product satisfactory. It is an especial feature to -make a rid leak in which the parts can be made largely by automatic machinery and assembled with accuracy without skill and with little labor.

To this end the invention provides a cup, being preferably a cylindrical shell closed at one end, and there constituting one terminal, within which cup is a hollow cylindrical resistance pad through the thickness of which the electrical circuit passes to the shell .from a cylindrical, expansible compressor within.

The latter is conveniently formed by cutting slots along a tube of resilient metal, deeply from one end thereof, thus converting its walls into spring sectorsand providing at the mouth of this element a cam for spreading these sectors radially. The cam may be a loose steel ball operated bya screw extending in the axial direction for forcing it into the mouth of said element, as far as desired, thus expanding the sectors radial y against the 5,0 said resistance medium, which latter is of such a nature that its resistance decreases as its",compression=increases. A composite cap, .felofsing the entrance to the cup, provides this i' "scr'gv element andan exterior terminal in- '.55', guinea ampie Shui body of the cup. Preferably, it is so arranged that an insulating bushing screws into the shell and supports interiorly a metallic part having an axial screw with interior plunger head to` ress the ball, and with exterior operating ead or handle. A loose inside piece may intervene between the screw and the ball, to transmit thrust of the screw to the ball. This thrust, converted to radial pressure by its cam expansion of the compressor, compresses the cylindrical resistance sheet against the cylindrical interior of the shell. By the construction described the compression may be very considerable, and will be approximately uniform. The current passes from the cap terminal through the screw, plunger, transmitter, compressor, resistance and shell to the shell terminal. y t

The construction thus described is ex-l tremelyn simple mechanically lrequiring but few operations of manufacture, and these can be performed by automatic machinery; while its results have been found verysatisfactory electrically. The resistance pad may be made of absorbent material such as cotton cloth, or blotting paper, in and among the libres of which are finely divided deposits of conducting material, as carbon or metallic silver. For example, blotting paper may be soaked in a carbon suspension ink, and dried or in silver ammonium nitrate and then f dipped in formaldehyde to precipitate metallic silver. When cleaned and dried, and insulated in the apparatus described, a resistance results which can be adjusted by screw pressure, with satisfactory accuracy through a wide range; as, for example, in the particular apparatus herein more specically described, a range of from one-half megohm to fifteen megohms has been found easily attainable.

The invention may be 'applied invarious ways, one of which is illustrated in the accompanying drawings and description. It is intended that the patent shall cover, by suitable expression in the appended claims, whatever features of patentable novelty exist in the invention disclosed.

In the drawings:

Figure 1 is a side elevation of a preferred form of grid leak;

Figure 2 is a medial longitudinal section of the same;

Figure 3 1s a transverseA section through the apparatus on line 3-3 of Figure 2;

Figure 4 is an elevation showing the grid leak mounted between conductor terminals, on a base of ordinary type; and

Figure 5 is an enlarged side elevation of a detail of Figure 2.

. `Referring to the drawings, a metal shell or cup 1 is represented, conveniently made on a screw machine from a brass rod by drilling a chamber 11. The exterior is smooth and adapted to be mounted on an ordinary base 4, having an exterior screw 14 and washer 13 for clamping one clip 12 of such a. base. Thus this shell 1 becomes one terminal of the grid leak. The mouth of the shell or cup is lilled by a ca or plug, one part of which 2 is of brass an constitutes the other terminal, while another part 20 is of rubber, a phenolic 'condensation product, or 'the like insulator and supports this terminal 2 in the mouth of the cup, together with an axial screw with external insulating handle 25. In Figure 4 the part 2 is seen held by the other clip 24,

v of the base 4. The inner end of screw 25 reaches into an axial cavity 26 which is open into the interior of the cup. This cavity 26 houses the stem of a loose sliding plunger or follower 27 whose Somewhat larger head is hollowed to adapt it for engaging a vsmall steel ball 30 within the cup .1.` The insulatin portion 2O of the cap is preferably an annuar bushing fast on the metal part 2 of the cap, having an external screw thread for engaging the screw thread in the .mouth of the cup 1 and lreinforced by an .internal steel collar 31 upon the edge of which the end of the tubular exterior 2 of the part 2 is turned over. Thus there is provided aclosed cylindrical chamber 11, with its body 1 constituting one terminal and its closure 2 constituting the other, and there is an axial screw 25 in the latter, adjustable from without, for thrusting the ball 30 more deeply into the chamber, or for letting it recede, and for connecting it electrically with the terminal 2.

Within the chamber is an insulating disc 15 at the bottom of the chamber; a high resistance hollow' cylindrical pad 16; and a `metallic annular compressor 17 adapted to compress the pad against the shell walls whenever it is expanded by the thrusting of ball 30 into its mouth. The compressor is conveniently made with four separated sectors 17a by slitting a tube or solid rod of sring brass lby sawcuts 17h deeply from one en throughout thegreater part of its length. But at its other end a portion of metal 17c is left for .holding the sectors all together. The free ends 17d of the sectors being set so as to have aslight circumferential marginal clearance,-

and being centrally countersunk, as seen in Figures 2 and 5, so as to present a. conical bearing surface against which the ball 30 e'ngageswhen it is thrust inward by the screw 25. In this respect the ball acts as a cam on the beveled eilds 17"i of the sectors. The sectors are thus arranged'somewhat as lagging round the intcrior'of the pad 16 and are capable of being expanded to apply pressure radially outward on this cylindrical pad. The ball is automatically centered both by these endsof the sectors and by its own follower 27, which has a cup shaped head. Th'us the pressure of the sector arms 17a is applied in approximately uniform manner radially outward, upon the sheet or pad of resistance material 16 compressing it against the cylindrical Wall of the cup 1 with a pressure which may be relatively great considering the smallness and simplicity of the parts by which it is generated, and which will remain permanently at whatever degree -it may be set. Upon retraction of the screw 25, the elasticity of the compressor arms'l?al will spring them back again toward the axis, so far as the retraction of ball 30 permits, thus relieving ticles are forced closer together thus decreas- A ing the electrical resistance of the sheets to currents passing through its thickness. Another method of preparing the resistance pad is to impregnate a. pad of fibrous material such as cotton or wood pulp blotting paper with a liquid from which subse uently a Inetallic recipitate is formed in nely divided condition distributed through the mass of the pad. An example of this is to impregnate the. pad with silver ammonium nitrate. A saturated shcet is then dipped into' a solution of formaldehyde which precipitates pure metallic silver in minute distributed particles throughout the body of the bed. Otherproducts of the reaction having then been washed .out in water the pad is to be dried and then is material which has veryl high resistance, and

yet by suitable design and adjustment to control the results with considerable accuracy.

When using the invention for resistances of lower order suitable modifications will naturally be made. When only a fixed resistance of specified value is wanted, such can be made by the described invention, and after being adjusted to the proper value the knob 25 can be cut off, leaving it practically inipossible to change the adjustment.

I claim:

1. A resistance unit comprising a cylindrical sheet of resistance material whose radial thickness constitutes the lengthwise dimension of the resistance, and compress/ion means acting radially thereon.

2. A resistance unit comprising a shell; a sheet of resistance material whose radial thickness constitutes the lengthwisedimension of the resistance, lining its interior; and means compressing the'sheet radially against the shell. l

3. A resist-ance unit comprising a shell; a sheet of resistance material whose radial vthickness constitutes the vlengthwise dimension of the resistance, lining its interior; an

expansible compressor within; and manually operative means for expanding the compressor against the resistance material and shell.

4. A resistance unit comprising acylindrical shell; a cylindrical sheet of resistance material within it; lagging within said cylindrical sheet; a circular cam'; and means acting through the cam Vto thrust the lagging radially outward thereby compressing said sheet against said shell.

5. A resistance unit comprising a cylindrical shell; a cylindrical sheet of resistance material within it; lagging within said cylindrical sheet; a circular cam; and means acting through the cam to thrust the lagging radially outward thereby compressing said sheet against said shell; and connections tor makingv the circuit through the cam and lagging.

6. A resistance unit comprising a cup and a plug closure therefor, these two constituting electric terminals and being insulated from each other; a sheet of resistance material within and lying around the side of the shell; and screw operative means for connecting the resistance 'electrically and compressing it mechanically between the plug and the cup. v

7. A resistance unit comprising-a cylindrical shell ot' condiu'zting material', a plug closing one end of it having a terminal insulated from it; and an interior connection from said terminal to the sidel of the shell through a compressive resistance.

` 8. A resistance unit comprising'a cylindrical shell of conducting material, a plug clos- "iilgonelend of it having a terminal insulated from it; an interior connection from said terminal to the shell through a compressivel resistance, comprising a screw and expanding element cooperating therewith and a resistance element lining the side of the shell and arranged to be pressed by the expansion of the said expanding element.

9. A resistance unit comprising a shell and resistance sheet liningv its interior; spring lagging within the sheet; a ball cam bearing against the ends of the spring lagging; and

ascrew adapted'to push the ball axially and .axial thrustr and to act with radial thrustfor compressing the resistance against the circular terminals.

12. A resistance unit comprising in combination two terminals; a resistance of compressive material; a. moving part on one of the terminals adapted lby cone and a cam connection, wherein the cone and cam are engaged -together to produce a spreading motion, to apply pressure tothe resistance material, to Vary its resistance; the said Vresistance material and the main course of flow through it being positioned laterally of the said cone. l b

13. A resistance unit comprising in combination terminal elements one of which is tubular and a tubular pad having finely divided conducting material disseminated through it so that the whole is of high resistance, variable by varying the degree of compressing of the pad, within said tubular terminal; and means to compress said tubular pad and tubular element together radially in tubula contact.

14. A grid leak comprising in combina-l tion terminal elements and a pad having finely divided conducting material disseminated through it, of which the resistance is variable by varying the degree of compressing of the pad; in which the compressive material is a mat consisting mainly of wood pulp in the nature of blotting paper. Signed at Boston, Massachusetts, thi third day of February, 1925. f

d sTnATFoRD B. ALLEN.

@ERTIFICATE 0F CORRECTION.

Patent No. 1,704,159. vGranted March 5, i929, to

i srRATFoRD B. ALLEN.

lt is hereby certified that error appears in the printed specification of the above numbered patent requiring correction as follows: Page 3, line 56, claim 7, strike out the word "and"; same page and claim, line 58, after the word "resistance" and before the period insert thesemi-colon and words material constituting said resistance; and means for compressing'said material"; andthat the said Letters Patent should be read with these corrections therein that the same may conform to the record of the ease in the Patent Office.

Signed and sealed 4this 18th day of June, A. D. 1929..

M. J.- Moore,l (Seal) Acting Commissioner of Patents.

CERTINGATE 0E? CRREGHN.

Patent No. 1,704,59. Granted Marsh S, i929, to

STRATFRD B. LLLEN.

it is hereby certifiad tha error appears in hepine speccaen of the above numbered paten requing correction as oows: Page 3, Eine S6, daim 7, strike out the word "and"; same page and dan, Eine 58, after he wd "resistance" and before the period inseft fab@ semi-colon @mi Words maerial cnstuing said fesistance; and means im" @@mesing said maera; and that the said Leters aem shuld be bead wih hese errecions therein that the same may ccnom to tbe record tbe case i h@ Faem fice.

Signed and sealed his 18m day @i me, A. El. 5.929.

M. I. Mame, (Seal) Acting; Commissioner o Patens. 

